Radium retention by blended cement pastes and pure phases (C-S-H and C-A-S-H gels): Experimental assessment and modelling exercises

Cementitious materials are planned to be used in current designs of geological disposal of radioactive waste. Their main function is the structural reinforcement, but they could contribute to the retention of some radionuclides via adsorption and/or precipitation. In this work, a set of laboratory experiments has been performed to determine the capacity of blended cement (CEM V/A) containing fly ash (FA) and blast furnace slag (BFS) to adsorb radium. The study has focused on the hydrated cement paste (HCP) with different degradation states and on cement representative pure phases (C-S-H and C-A-S-H at variable Ca/Si and Al/Si ratios). The experimental results show that retention of Ra by hydrated cements is high; C-S-H and C-A-S-H phases were proven to be very relevant in Ra uptaking, with log Kd up to 4.2 and 5.1, respectively. The adsorption of radium into these phases is interpreted to be driven by surface complexation on weak and strong silanols-like sites and ionic exchange with Ca. Since Ra adsorption has been proven to be inversely proportional to dissolved Ca content, an enhanced adsorption capacity of the cementitious hydrates (C-S-H phases) is expected as degradation of cement proceeds.